In Norse mythology, Alsvid ("all-swift") was one of the horses (with Arvak) that pulled Sol's chariot (i.e. the sun). The horses' mane gave off the light; the sun itself gave off the heat.
Explanatory Quotes
From "Norse Myths: Gods of the Vikings" re-told by Kevin Crossley-Hollands (Penguin, 1982):
From The Creation
"Sun follows behind [the moon]. One of her horses is called Arvak because he
rises so early, and the other Alsvid because he is immensely strong. The Aesir
inserted iron-cold bellows under their shoulder blades to keep them cool. Sun
always seems to be in a great hurry, and that is because she is being chased by
Skoll, the wolf who is always snapping and growling close behind her."
From The Lay of Grimnir
"Arvak the Early Waker and Alsvid, All Swift, are the names of the steeds
whose wearisome work is to drag the sun across the sky. Long ago the gods took
pity on them, and put bellows under their yokes. And in front of the sun, like
a shield, stands Svalin. Should he let his guard slip, the mountains and the
sea would burst into flames. Skoll is the wolf on the tail of the sun, and he
will chase her until at last he runs her down in Iron Wood."